In recent years, an electronic technique called “spintronics” has been brought into the spotlight. While the conventional electronics uses “electric charge” that is one property of an electron, the spintronics, in addition to this, also actively uses “spin” that is another property of an electron. Particularly, “spin-current” that is flow of spin angular momentum is an important concept. Since energy dissipation of spin current is small, there is a possibility that using spin energy can accomplish highly efficient information transfer. Accordingly, generation, detection, and control of spin current are important themes.
For example, there is known a phenomenon that when an electric current flows, spin current is generated. This is called the “spin-Hall effect”. As a phenomenon opposite thereto, it is known that when spin current flows, electromotive force is generated. This is called the “inverse spin-Hall effect”. By using the inverse spin-Hall effect, spin current can be detected. The spin-Hall effect and the inverse spin-Hall effect are significantly appeared in a material (e.g., Pt or Pd) whose “spin orbit coupling” is strong.
By recent research, existence of “spin-Seebeck effect” in a magnetic material has been made clear. The spin-Seebeck effect is a phenomenon that when a temperature gradient is applied to a magnetic material, spin current is induced in a direction parallel to the temperature gradient (for example, refer to the patent literature 1 and the patent literature 2). In other words, by the spin-Seebeck effect, heat is converted into spin current (thermal spin-current conversion). Patent literature 1 reports the spin-Seebeck effect in a NiFe film that is a ferromagnetic metal. Non-patent literatures 1 and 2 report the spin-Seebeck effect at an interface between an electromotive film and a magnetic insulator such as yttrium iron garnet (YIG, Y3Fe5O12).
Spin current induced by a temperature gradient can be converted into an electric field (electric current, voltage) by using the above-mentioned inverse spin-Hall effect. Namely, using both of the spin-Seebeck effect and the inverse spin-Hall effect enables “thermoelectric conversion” that converts a temperature gradient into electricity.
FIG. 1 illustrates a configuration of a thermoelectric conversion element disclosed in the patent literature 1 and using the spin-Seebeck effect. On a sapphire substrate 101, a thermal spin-current conversion unit 102 is formed. The thermal spin-current conversion unit 102 has a layered structure of a Ta film 103, a PdPtMn film 104, and a NiFe film 105. The NiFe film 105 is a magnetic film having magnetization in an in-plane direction. Further, on the NiFe film 105, a Pt film 106 is formed as an electromotive film. Both ends of the Pt film 106 are connected to terminals 107-1 and 107-2, respectively.
In the thus-configured thermoelectric conversion element, the NiFe film 105 plays a role of generating spin current from a temperature gradient by the spin-Seebeck effect, and the Pt film 106 plays a role, which is a role as a spin current to electric current conversion material, of generating electromotive force from the spin current by the inverse spin-Hall effect. Concretely, when a temperature gradient is applied in the in-plane direction of the NiFe film 105, the spin current is generated in a direction parallel to the temperature gradient by the spin-Seebeck effect. Then, the spin current flows from the NiFe film 105 into the Pt film 106. Alternatively, the spin current flows out of the Pt film 106 to the NiFe film 105. In the Pt film 106, by the inverse spin-Hall effect, the electromotive force is generated in a direction perpendicular to the spin current direction and the NiFe magnetization direction. The electromotive force can be brought out from the terminals 107-1 and 107-2 provided at the both ends of the Pt film 106.
FIG. 2 illustrates a configuration of a longitudinal thermoelectric conversion element disclosed in Patent Literature 2. As illustrated in FIG. 2, an electromotive layer 120 is stacked on a magnetic layer 110. In the case of the longitudinal thermoelectric conversion element, a temperature gradient is applied in the stacking direction.
When a temperature gradient is applied in the stacking direction, thermal spin current is generated in the same direction, i.e., from a high temperature side to a low temperature side. Further, at an interface between the magnetic layer 110 and the electromotive layer 120, the thermal spin current undergoes a process called spin injection, and generates pure spin current into the electromotive film. The spin injection is a phenomenon in which a spin that performs precession adjacent to the interface around an axis of a magnetization direction interacts with a conduction electron that exists in the electromotive film and that does not have a spin, and gives spin angular momentum to the conduction electron, or receives spin angular momentum from the conduction electron. As a result, in the electromotive layer 120, near the spin injection interface, “pure spin current” by conduction electrons having spins is generated. In this pure spin current, an up-spin and a down-spin current in directions opposite to each other, and for this reason, an electric charge does not move in the direction of the pure spin current, and only momentum of spins flows.
When the electromotive layer 120 is formed of a material having large spin orbit coupling, electromotive force is generated in the direction perpendicular to the spin current direction and to the magnetization direction by the inverse spin-Hall effect.
In the spin current thermoelectric conversion element as described above, magnitude of generated electromotive force is obtained by multiplying amount of spin current generated in the magnetic layer by spin current injection efficiency (injection efficiency of spin current at the interface with the electromotive layer) and spin current to charge current conversion efficiency (efficiency at which spin current is converted into electromotive force by the inverse spin-Hall effect in the electromotive layer). Accordingly, three indexes of amount of spin current itself, spin current injection efficiency, and spin current to charge current conversion efficiency need to be increased at the same time for obtaining a thermoelectric conversion element having larger output. Improvement in spin current to charge current conversion efficiency in the electromotive layer among them is an important theme for other spintronics elements as well.
A material of the electromotive layer has both an electrical conductivity and the spin-Hall conductivity. A dimensionless index representing the spin-Hall conductivity and an electrical conductivity is called “spin-Hall angle”. The spin-Hall angle is used as an index for magnitude of the spin-Hall effect.
In typical experiments, pure Pt having the large spin-Hall angle is often used as the electromotive layer. Although the spin-Hall angles of Au, Ag, Cu and the like as similar pure noble metals fall short of that of Pt, there is a case where the spin-Hall angle larger than that of pure Pt can be obtained by introducing a minute quantity of Fe introduced as impurities into Au, or adding Ir to Cu, for example.